Another Alpha bug in Resolve

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dave4443

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Another Alpha bug in Resolve

PostTue Nov 23, 2021 12:53 pm

I'm not sure exactly why Resolve has a tough time using footage with alpha layers from After effects and always changes the way they look, but again I have here in After effects some text with a drop shadow on it and then the same text used in Resolve

Can anyone please explain why this would happen? I'm almost certain this is just a bug in Resolve since there's nothing I could really be doing wrong here, I tried exporting it in different formats and it just doesn't work properly when placed in Resolve
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dariobigi

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Re: Another Alpha bug in Resolve

PostTue Nov 23, 2021 1:43 pm

Have you tried changing the clip attributes? Toggle straight or pre multiplied


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dave4443

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Re: Another Alpha bug in Resolve

PostThu Nov 25, 2021 3:11 pm

dariobigi wrote:Have you tried changing the clip attributes? Toggle straight or pre multiplied


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When it's in Alpha mode None, it looks like it should, except obviously there is no alpha channel here, when it's on straight the shadow turns white, it makes 0 sense...

It definitely appears to be a bug as I exported some text from After effects today in a different composition and it works as intended, Resolve just appears to sometimes work and sometimes not work when it comes to alpha layers
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dariobigi

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Re: Another Alpha bug in Resolve

PostThu Nov 25, 2021 4:23 pm

Bugs aren’t necessarily inconsistent. It’s all just math. I suggest doing a deep detailed documentation of settings on both apps and renders to isolate the cause. Only then will BMD support will respond with a reason or fix. Don’t forget to include all your system info and both app versions as they request in their bug reports. (Pinned at the top of the forum.)


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dave4443

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Re: Another Alpha bug in Resolve

PostFri Nov 26, 2021 3:38 pm

dariobigi wrote:Bugs aren’t necessarily inconsistent. It’s all just math. I suggest doing a deep detailed documentation of settings on both apps and renders to isolate the cause. Only then will BMD support will respond with a reason or fix. Don’t forget to include all your system info and both app versions as they request in their bug reports. (Pinned at the top of the forum.)


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I do appreciate the response, but last time I went down a fully detailed route with the BMD workers on here, sending project files, reports, footage etc. It ultimately just got ignored and then they stopped responding to my messages when I chased it up, so I just stick with the community quick answers now, usually to see if anyone else has the same issue and what they did to get around it, because most of my bugs happen on multiple machines at home and at work which tends to point towards the software
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dariobigi

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Re: Another Alpha bug in Resolve

PostFri Nov 26, 2021 4:56 pm

Sorry to hear that. I’ve had several BMD support interactions over the years and had helpful information come back to me. Unfortunately, knowing the bugs and workarounds of software has always been the case for my 20+ years in post production. I can say BMD has been better than most in resolution and improvements. Only surpassed by Nobe OmniScope support, in my opinion. Best of luck. Patience & Perseverance unto you and us all.


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Annaël Beauchemin

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Re: Another Alpha bug in Resolve

PostFri Nov 26, 2021 10:54 pm

dave4443 wrote:I'm not sure exactly why Resolve has a tough time using footage with alpha layers from After effects and always changes the way they look, but again I have here in After effects some text with a drop shadow on it and then the same text used in Resolve

Can anyone please explain why this would happen? I'm almost certain this is just a bug in Resolve since there's nothing I could really be doing wrong here, I tried exporting it in different formats and it just doesn't work properly when placed in Resolve


It looks like your alpha is premultiplied with white. Can you try to export using straight alpha instead?

If your AE comp's BG is set to white, I think it changes the premult color. If exporting straight doesn't work, you could try to switch your comp to black and export again in premultiply.
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Re: Another Alpha bug in Resolve

PostSat Nov 27, 2021 9:38 am

Annaël Beauchemin wrote:It looks like your alpha is premultiplied with white. Can you try to export using straight alpha instead?

Since this is an area of great confusion and time waste, lets be precise. Alpha values are not premultiplied with anything, RGB values are. And they aren’t premulted with a color either, instead they are multiplied with alpha and merged with a set color background and thus essentially contaminated. Values can be restored within limitations of bit depth precision because composite value is known, background value is known and alpha is known so fg can be recalculated from over equation.
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Re: Another Alpha bug in Resolve

PostSat Nov 27, 2021 10:48 am

For anyone unsure about the alpha channel, I really liked Simon Ubsdell's videos on this, which are two parts of his five-part series called "The Basics of Compositing Theory". Here's the two videos on alpha





Well worth a watch IMHO for anyone who's not already an expert. And all demonstrated in Fusion.

One of the video mentions how AE is unusually smart in detecting when imported media is 'pre multiplied' over a non-black background - he shows it detecting a yellow background.
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Annaël Beauchemin

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Re: Another Alpha bug in Resolve

PostSat Nov 27, 2021 7:04 pm

Hendrik Proosa wrote:
Annaël Beauchemin wrote:It looks like your alpha is premultiplied with white. Can you try to export using straight alpha instead?

Since this is an area of great confusion and time waste, lets be precise. Alpha values are not premultiplied with anything, RGB values are.

Thank you for correcting my wordings. You are right, and using the correct words should help fellow users to understand what is going on with the pixels when we use premultiplied RGBA image.

Still, lots of programs do not make the distinction that it's the color values that get multiplied and not the alpha. I assume this is done to simplify the text. Also, correct me if I am wrong, but the "Premultiplied" Alpha mode in Resolve should be written "Premultiplied with Black".

So let me rephrase my answer to the OP:
In AE, when you export an alpha with Premultiplied color values, the background color of the composition gets multiplied. In Resolve, setting the Alpha mode to "Premultiplied" assumes that the color of the premultiplication was black. So if your background what anything but black in AE, Resolve will not unmultiply the color values correctly when it is composited in Resolve. So you should either export in Straight alpha (my personal favorite since this is the default of Resolve), or you should set to black the background color of your AE composition and re-render with premultiplied color values.

How does that sounds?
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Re: Another Alpha bug in Resolve

PostMon Nov 29, 2021 11:03 am

Annaël Beauchemin wrote:
Hendrik Proosa wrote:
Annaël Beauchemin wrote:It looks like your alpha is premultiplied with white. Can you try to export using straight alpha instead?

Since this is an area of great confusion and time waste, lets be precise. Alpha values are not premultiplied with anything, RGB values are.

Thank you for correcting my wordings. You are right, and using the correct words should help fellow users to understand what is going on with the pixels when we use premultiplied RGBA image.

Still, lots of programs do not make the distinction that it's the color values that get multiplied and not the alpha. I assume this is done to simplify the text. Also, correct me if I am wrong, but the "Premultiplied" Alpha mode in Resolve should be written "Premultiplied with Black".

So let me rephrase my answer to the OP:
In AE, when you export an alpha with Premultiplied color values, the background color of the composition gets multiplied. In Resolve, setting the Alpha mode to "Premultiplied" assumes that the color of the premultiplication was black. So if your background what anything but black in AE, Resolve will not unmultiply the color values correctly when it is composited in Resolve. So you should either export in Straight alpha (my personal favorite since this is the default of Resolve), or you should set to black the background color of your AE composition and re-render with premultiplied color values.

How does that sounds?



I appreciate the in depth response, but I tried to change all the settings like that and the result was the same sadly, and the after effects was exported in straight alpha which is why I'm unsure why this is happening with some of my text, but not other ones from the same after effects project haha
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Re: Another Alpha bug in Resolve

PostMon Nov 29, 2021 11:45 am

In AE were you compositing in Linear gamma?
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Re: Another Alpha bug in Resolve

PostMon Nov 29, 2021 12:17 pm

Can you upload your title file for inspection?

Annaël Beauchemin wrote:Still, lots of programs do not make the distinction that it's the color values that get multiplied and not the alpha. I assume this is done to simplify the text. Also, correct me if I am wrong, but the "Premultiplied" Alpha mode in Resolve should be written "Premultiplied with Black".

So let me rephrase my answer to the OP:
In AE, when you export an alpha with Premultiplied color values, the background color of the composition gets multiplied. In Resolve, setting the Alpha mode to "Premultiplied" assumes that the color of the premultiplication was black. So if your background what anything but black in AE, Resolve will not unmultiply the color values correctly when it is composited in Resolve. So you should either export in Straight alpha (my personal favorite since this is the default of Resolve), or you should set to black the background color of your AE composition and re-render with premultiplied color values.

How does that sounds?

Some thoughts, might help narrow down the problem area... "Premultiplied with Black" is kind of meaningless because if we look at what premult actually is, there is no multiplication with black anywhere. The classical over blend or so-called compositing operation (which is nothing more than linear interpolation between two values) is expressed as C = A * alphaA + B * (1.0 - alphaA). The premultiplication part is the A * alphaA and its reasoning is that since both A and alphaA are known at the time of storing the A image, this multiplication operation can be done beforehand, simplifying the compositing computation. So premultiplication is the multiplication of RGB channel values of image A with respective alpha of that pixel. Not multiplication with black, not multiplication with color. Alpha channel has no color, it is non-color data, and RGB channels express the image itself. Since alpha is zero where there is no "coverage", RGB values are scaled to 0.0 value for those pixels which looks black when viewed. Straight alpha simply means that this multiplication operation is not performed and RGB values are stored as-is.

Now, if we have an image, lets say white text on yellow backgound, and we have alpha channel for the text, we could say the image is matted with yellow in AE speak. This means that our text element has been already merged with yellow bg using the over operation above, so we have our C, and we have our alphaA. By acknowledging that our background is yellow we can also tell our software what B was. This allows to regain the A value: A = (C - (B * (1.0 - alphaA))) / alphaA. There is no practical benefit from compositing standpoint for doing this, it is always preferable to have just pure unpremultiplied image or premulted one (which can help with data compression due to zero alpha pixels being also zeroed in RGB channels). Historical reason why this is "a thing" is afaik that this "matted with color" option allows having the final composited graphics for nice viewing while still attaching the alpha too in case soneone wants to pick it apart again. And this in turn is the reason why some formats store RGB data in original not premultiplied form, png for example. Because multing would lose the bg and just leave the ugly color contamination fringe in semitransparent areas, not nice for the hovering art director. And even if image is actually premulted, knowing the bg color can still help in reproducing original values and thus removing the contamination. This property is the basis for some fg color restoration methods in keying for example.
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Re: Another Alpha bug in Resolve

PostMon Nov 29, 2021 3:54 pm

Here’s my understanding of straight and premultiplied. Let’s say you have an image of color bars with an alpha channel of all white (1.0) with a black (0.0) circle in the middle. If you stripped the alpha channel from the straight image you’d still have the full color bar image. If you stripped it from the premultiplied one, you’d have a black circle in the middle of the color bars.
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